1. Field of the Inventions
The present invention relates to mixed catalyst systems and bimodal polyethylenes made from these mixed catalyst systems; and more particularly, the present invention relates to a catalyst system comprising a cyclic bridged metallocene and a bridged metallocene, preferably supported, capable of producing a bimodal polyethylene.
2. Description of the Related Art
The polymerization processes described herein can be a solution, gas phase, slurry phase or High-pressure process. As discussed in greater detail below, gas phase or slurry phase polymerization processes are preferred, involving catalysts and olefin monomers, at least one of which is ethylene or propylene. As reflected in the patent literature, a great deal of effort has been expended towards discovering improvements in such processes. Some of those patents are identified herein. The improvements offered by the inventions described herein are set forth in greater detail below.
In particular, there is a desire to prepare bimodal polyolefins, especially bimodal polyethylene, using only one reactor as this requires less capital cost than using staged reactors. One method of producing bimodal polyolefins in a single reactor is by using mixed catalyst systems. While some types of mixed catalyst systems are known in the art, for example WO 98/49209; U.S. Pat. No. 4,530,914; U.S. Pat. No. 5,525,678; U.S. Pat. No. 6,271,323; U.S. Pat. No. 6,399,772; U.S. Pat. No. 6,410,474, many of these disclose processes of making bimodal polyethylenes having a relatively high density resin of at least 0.940 g/cm3, with a limited breadth of rheological (e.g., melt flow) properties. It would be desirable to expand on that range.
It has been unexpectedly found that by manipulating the way in which the mixed catalyst system is produced, the rheological properties of the polyolefins produced from those mixed catalysts may be tailored. The present invention is thus directed towards catalyst systems and method of producing catalyst systems having improved properties that expand the range of bimodal polyolefins that can be produced in a single reactor in a single step.